The present invention relates to a shift valve for an automatic transmission.
A representative example of conventional shift valves is illustrated in FIG. 1. This conventional shift valve for an automatic transmission comprises a spool 10 for switching an oil delivery conduit, a plug 12, and a spring 14 arranged between them. Governor pressure P.sub.G acts on the spool 10 in one direction and throttle pressure P.sub.TH acts on the plug 12 in another direction which is opposite to the one direction. With this shift valve for automatic transmission, a change-over characteristic as shown in FIG. 2, i.e., a shift schedule line, is obtained. This shift schedule line consists of a line portion A representing that governor pressure P.sub.G varies with throttle pressure P.sub.TH and another line portion B representing that governor pressure P.sub.G is unchanged irrespective of variation in throttle pressure P.sub.TH. If the force of the spring 14 is changed, the governor pressure value P.sub.G for the line portion B can be varied, but the line portion A remains unchanged. (In order to change the line portion A, a pressure acting area of the spool 10 or the plug 12 must be varied.)
Various kinds of shift schedule lines are required depending upon different types of engines and different types of vehicles. If, in order to provide a different shift schedule line, the valve bore, spool and plug are redesigned in configuration, numerous kinds of valve bodies, spools and plugs must be manufactured, complicating manufacturing of and management of component parts, and pushing up cost as well. Thus, it is quite advantageous if a different shift schedule line is provided only by replacing the spring. However, in the case of the above mentioned conventional shift valve for automatic transmission, replacing the spring will result in a change in only a portion of the shift schedule line. That is, with the conventional shift valve, it is not possible to vary the line portion A by changing the characteristic of the spring.